The present invention relates to impregnated diamond drill bits. Prior to the development of the invention disclosed in the above noted application, the entire crown structure of impregnated diamond drill bits were formed using a single metal matrix composition, which composition had a fairly high degree of hardness. The matrix material on the lower face of the crown structure had to be capable of wearing away in order to expose new cutting surfaces of the diamond particles. Since the same composition was used for the entire metal matrix, both the outside and inside side walls of the crown structure also had a tendency to erode during utilization of the drill. Such erosion in the formation of the drill bit construction actually changes the size and shape of the core being drilled. Furthermore, the portions of the walls which do erode away can impede the water circulation that travels across the surface of the crown structure and through the hollow center of the drill.
The above noted drawback becomes even more pronounced when using a metal matrix structure having a lower degree of hardness, such as on the order of 10 Rockwell C, as disclosed in the above noted application. The problem becomes especially significant where the material in which the drill is being used is extremely hard and abrasive.
In order to overcome these problems, it has been the practice to place surface set diamond particles in both portions of the inner and outer circumferential surfaces of the bit. Such surface set diamonds help to maintain the gauge of the bit by preventing erosion in those areas. While limiting the amount of erosion, the use of such surface set diamonds does not completely avoid the above-described difficulties. Furthermore, the use of such surface set diamonds provides an additional expense both for the cost of the diamond particles and for the cost of hand-setting those diamond particles in the mold when forming the impregnated bit.